The faint light was just enough to enable Raoul to distinguish the shape of things around him.

And he could not restrain a dull cry: there were three corpses there.
The first lay on the narrow landing of the little staircase; the two others had rolled to the bottom of the staircase.

Raoul could have touched one of the two poor wretches by passing his fingers through the partition.
"Silence!" whispered the Persian.
He too had seen the bodies and he gave one word in explanation: "HE!" The commissary's voice was now heard more distinctly.

He was asking for information about the system of lighting, which the stage-manager supplied.

The commissary therefore must be in the "organ" or its immediate neighborhood.
Contrary to what one might think, especially in connection with an opera-house, the "organ" is not a musical instrument.